The present invention relates generally to electrical generators employing superconducting rotors, and more particularly to a cooling system for supplying coolant at superfluid temperatures to a superconducting rotor.
In electrical generators having superconducting rotors, the windings on the rotor are immersed in a pool of very cold liquified gas, such as liquid helium. The liquid helium absorbs heat from the windings and consequently evaporates to its gaseous (vaporous) form. The rotor winding pool is in flow communication with a source of helium and to a pump which delivers liquid helium to the pool at a very low pressure, and hence at a superfluid temperature. Because of the very low pressure, air may be drawn into the rotating transfer system. When this occurs, the air freezes and can plug the cooling system and disrupt the proper operation of the system.
Superfluid helium is often produced and maintained by vacuum pumping liquid helium to low pressure inside a dewar. Minimization of heat leaks is important because the vacuum pumps that produce the low pressure over the superfluid are operated at room temperature and become physically very large in order to pump significant quantities of low pressure, high specific volume helium vapor.
Applicant has recognized that if a rotor cooling system could be implemented with a vacuum pump that would operate reliably with very low temperature helium, then the pump could be integrated within the rotor and thereby eliminate external pump connections and their inherent air leak problems. Also, at low temperature the specific volume of helium vapor is much smaller and the pump and associated piping could be made much smaller. However, reliable operation of the moving parts of conventional pumps at such low temperatures of liquid helium has not been accomplished with any acceptable degree of reliability.